1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a distributed-feedback laser with improved analog modulation distortion characteristics and a method for fabricating the same.
2. Description of the Related Art
A low-distortion distributed-feedback laser is demanded for an analog modulation light source for use in a subcarrier multiplex optical transmission such as a CATV (Cable Television) system. Intermodulation distortion of the distributed-feedback laser is greatly influenced by the linearity of current versus optical output characteristics of the laser. In a conventional distributed-feedback laser, the linearity of the current vs. optical output characteristics is poor due to nonuniformity of the longitudinal electrical field distribution in the laser cavity and, for this reason, it is difficult to obtain satisfactory distortion characteristics for analog transmission. Because the device characteristics are greatly influenced by the grating phases at both cleaved facets, there have been large difference in the characteristics in individual devices, and this has made it difficult to obtain stable single-mode operable devices with high production yield.
A conventional DFB (distributed-feedback) laser is mostly and primarily for use in digital modulation, so that there has developed a .lambda./4 phase-shift DFB laser having a phase shift region in the center of grating and asymmetrical coatings for making a large gain difference large between a main mode and a sub-mode. However, even in such a .lambda./4 phase-shift DFB laser, there has been a problem of the occurrence of two-mode operation due to spacial hole burning, i.e., a nonuniform carrier density distribution resulting from a nonuniform electrical field distribution.
As a means to attempt to overcome the above problem, there has been a proposal wherein changes are made in the depths, shapes or widths of groves of diffraction gratings along the active region (that is, in the direction in which light propagates). Examples of such a proposal are found in Japanese Patent Application Kokai Publication Nos. H2-90688, H2-172289, H3-110885 and H2-20087. Japanese Patent Application Kokai Publication No. H2-281681 discloses an example wherein an attempt has been made to control the coupling constant to attain single-mode operation by changing stripe widths of an optical waveguide layer having the diffraction gratings.
In the conventional DFB laser, a phase shift region is provided at a portion of the diffraction grating (usually in the neighborhood of the center thereof) and, due to the occurrence of concentration of electric fields at that portion, there has occurred spacial hole burning. In order to reduce the occurrence of such spacial hole burning, it has been necessary to make a precise control of the shapes and widths of the groves or corrugations of the diffraction grating, and this has presented problems relating to manufacturing precision and production yield.
Also, in the conventional DFB laser, since it is not intended for analog modulation and no consideration is given to the linearity of current vs. optical output, the characteristics as required for analog modulation have been poor.
In the conventional DFB laser or phase-shift DFB laser having asymmetrical coatings, electrical field distribution along the cavity is nonuniform, which makes the linearity of current vs. optical output poor and renders such DFB laser unsuitable for use in analog optical communication.
The DFB laser for use in analog optical communication requires severe low distortion characteristics. Especially, for example, in the DFB laser used in the 42 channel cable television (CATV) networks, the low distortion characteristics, such as CSO (composite second-order distortion) .ltoreq.-60 dBc and CTB (composite triple beat distortion) .ltoreq.-65 dBc must be satisfied. This requirement is not met by the conventional DFB laser as described above.